Pumps are critical components in wastewater treatment facilities. The Sanford (Maine) Sewerage District experienced just how critical when the pumps feeding biosolids to the centrifuges at its wastewater treatment plant began to fail.

The district, serving a community of 21,000, has been in operation since 1947. The treatment plant (4.4 mgd design, 1.56 mgd average) is fed by 70 miles of gravity sewers and 16 pumping stations. It also treats the septage from surrounding towns.

The activated sludge oxidation ditch treatment process includes biological nutrient removal with chemical addition for additional removal of phosphorus. This is followed by multimedia filtration (Roberts Filter Group) to reduce phosphorus still further. After UV disinfection (Trojan Technologies), the tertiary effluent is discharged to the Mousam River, which has a dam impoundment several miles downstream of the outfall. 

On the solids side, waste activated sludge is directed to holding tanks at 0.5-0.9% solids. The material is dewatered to 20% solids in two centrifuges (GEA Group) with a capacity of 800 pounds per hour at 165 gpm per machine. The dewatered biosolids are sent to the district’s compost facility and ultimately landfilled.

TROUBLE WITH PUMPING

The delivery of sludge to the centrifuges leaves no room for pump failure. So, when the two aging centrifuge feed pumps experienced excessive wear on seals, rotors and stators, it was time for action.

“It became apparent that we needed to do something with our pumps,” says Scot Lausier, chief plant operator. “It became costly with repairs and parts needing to be replaced constantly, and our pumps began to experience downtime because the replacement parts had long lead times.

“This is not something you want happening when you are the wastewater facility protecting the waterways in your own and surrounding areas. Our customers are counting on us. They trust us. We can’t have our pumps failing. We can’t let our community down.”

SEEKING A SOLUTION

The district needed new pumps with technology able to handle grit with less susceptibility to wear and tear. “We couldn’t risk any more setbacks or failures,” says Lausier. “We needed to look at our issue in a different way because replacing the pumps with units like the ones we had been using wasn’t going to solve the problem in the long run.” The district evaluated several types of pumps but did not find a model the staff trusted to withstand the harsh environment they would be exposed to, day after day.

Among the variety of pumps used in the plant for transfer of liquids, the district had been using a septage transfer pump from Penn Valley Pump for some time. The pump had capacity to transfer millions of gallons of septage with no issues. Knowing this, Lausier contacted Penn Valley Pump to explore replacement possibilities.  

The company suggested its Double Disc Pumps, engineered for an extremely low wear rate with a low-friction design and able to last thousands of hours between rebuilds. At first, Wright-Pierce engineers were concerned about possible sludge feed pulsation from that type of pump, but after installation that concern was nullified.

SUITED TO THE TASK

“The Double Disc Pumps were a perfect option for us,” says Lausier. “There was a local representative from the company who could answer any questions or concerns we had. There were also replacement parts at the ready if we ever needed them.

“The pumps are easy to maintain because of their maintain-in-place hinged housing that allows for quick and straightforward disassembly and reassembly. Each pump only has five elastomeric components and a gasket set. That makes it quick and easy to get them back into service if they ever go down.”

The district has used the pumps for more than two years. “We have had zero maintenance issues,” says Lausier. “Pump 1 has 4,500 hours of run time, and Pump 2 has 3,500 hours. We could not be happier.”

By replacing the sludge pumps, the district continues to operate a reliable, natural and efficient process of purifying wastewater and protecting local water resources. 

Preston Campbell (pcampbell@pennvalleypump.com) is applications engineering director at Penn Valley Pump.